TSQL Datediff(dd,MAX(p_effdt),CURRENT_TIMESTAMP)

Question

Hi,
OBJECTIVE: for a group of clients, report the average number of days in which they have been enrolled in a given program.
PROBLEM: the clients are often dropped and later re-enrolled in our programs.  So, I want to use only the most recent enrollment date for each client.  And then calculate an average number of days enrolled.
The data looks like this:
c_id p_effdt p_lapdt
123456 12/19/2013 NULL
234567 11/10/2003 12/23/2003
345678 11/25/2010 NULL
345678 12/16/2010 4/29/2010
456789 11/11/2004 11/30/2004
456789 11/13/2003 12/29/2003
Of course, this: Datediff(dd,MAX(p_effdt),CURRENT_TIMESTAMP)
results in the error: Cannot perform an aggregate function on an expression containing an aggregate or a subquery.
DECLARE @LastProgramEffectiveDate DATETIME
SET @LastProgramEffectiveDate = MAX(program.p_effdt)
SELECT AVG(Datediff(dd,@LastProgramEffectiveDate,CURRENT_TIMESTAMP))
FROM events INNER JOIN program ON events.e_case_no = program.c_id
Error: The multi-part identifier "program.p_effdt" could not be bound.
So that SQL query is not the way to go.  What is the best way to accomplish the above defined objective?
Thank you.
Alpineer8

Answer 1

It's not clear what you are doing.  When you do a MAX(), you only get one value per group (or if your query doesn't have a GROUP BY, one value per query).  It makes no sense to do an AVG of one value - it is just that value.  So, I suspect
there is something more complicated about what you are doing.  Could you provide a more complete description of what you are doing, and sample tables and sample data and the result you want from that sample data?  Also, let us know the release of
SQL Server you are using.  Doing that usually lets us quickly provide you with a helpful answer.
Tom